Simple server you can run on Port 443 that displays incoming client certs

https-server.go

package main

import (
	"crypto/tls"
	"crypto/x509"
	"fmt"
	"github.com/grantae/certinfo"
	"io/ioutil"
	"log"
	"net/http"
)

func HelloServer(w http.ResponseWriter, req *http.Request) {
	w.Header().Set("Content-Type", "text/plain")
	w.Write([]byte("This is an example server.\n"))
	if req.TLS != nil {
		for _, crt := range req.TLS.PeerCertificates {
			result, err := certinfo.CertificateText(crt)
			if err != nil {
				log.Fatal(err)
			}
			fmt.Print(result)
		}
	}
}

func main() {
	log.SetFlags(log.Lshortfile)

	cachain_b, err := ioutil.ReadFile("cacert.pem")
	if err != nil {
		log.Println("Can't read cachain file: %v", err)
		return
	}
	pool := x509.NewCertPool()
	pool.AppendCertsFromPEM(cachain_b)

	tlsConfig := tls.Config{
		ClientAuth: tls.RequireAndVerifyClientCert,
		ClientCAs: pool,
	}

	server := &http.Server{
		Addr:      ":4443",
		TLSConfig: &tlsConfig,
	}

	http.HandleFunc("/", HelloServer)
	err = server.ListenAndServeTLS("../my-smtp-proxy/fullchain.pem", "../my-smtp-proxy/privkey.pem")
	if err != nil {
		log.Fatal("ListenAndServe: ", err)
	}
}